President Trump offered full backing Tuesday to the United Kingdom as it investigates the poisoning of a former Russian spy and his daughter with a nerve agent, and called on Russia to provide “unambiguous answers” on the subject.

Prime Minister Theresa May spoke with Trump about the incident, according to readouts from both the White House and 10 Downing Street, and reiterated that it was “highly likely” that Russia was responsible for the poisoning of Sergei and Yulia Skripel in Salisbury.

“President Trump said the U.S. was with the U.K. all the way, agreeing that the Russian government must provide unambiguous answers as to how this nerve agent came to be used,” a Downing Street spokesperson said.

May told lawmakers in the House of Commons Monday that the pair were exposed to a nerve agent called Novichok and also said there that it was "highly likely" that Russia was responsible.

“This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals, it was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk,” May said.